Two of the most basic building blocks in power electronics are the buck and the boost converter circuits 100, 200 illustrated in FIGS. 1 and 2, respectively. Each of these converter circuits 100, 200 generally include input nodes 104A-B, 204A-B, output nodes 108A-B, 208A-B, an inductor 112, 212, a diode 116, 216, a smoothing capacitor 120, 220 across the high voltage nodes and a switch 124, 224. The operation of buck and boost converter circuits 100, 200 is well-understood by virtually all power electronics engineers. Typically the switches 124, 224 in these circuits 100, 200 are turned on and off at a constant frequency and with an adjustable duty factor. The duty factor is used to control the input-to-output voltage ratio. Buck and boost circuits 100, 200 can be combined to make the half-bridge circuit 300 shown in FIG. 3 that includes two pairs each consisting of a diode 304, 308 and a switch 312, 316. Half-bridge circuit 300 is a buck converter when current and power are flowing left to right (relative to FIG. 3) and a boost converter in the opposite direction.
Devices used for switches 124, 224, 312, 316 in these switching power converter circuits 100, 200, 300 have included MOSFETs, IGBTs, Bipolar Transistors, GTOs, MCTs, and other power switches that can be turned on and off quickly and relatively easily with minimal power loss and high reliability. All of these devices have some power losses. Because of this power loss, all of these devices cause a limit to the amount of power that can be converted in a specific application.
For higher power applications, it is common for designers to use parallel switching devices to spread the losses in the switches. FIG. 4A illustrates a buck converter circuit 400 having parallel switches 404, 408. Of course, a greater number of switches can be implemented. Two are shown for simplicity. Paralleling the switches in this manner allows better cooling because the heat is spread out. Higher efficiency is also possible. When parallel switches are used in this fashion, all the parallel switches are turned on and off at the same time, thereby acting as one larger switch. There are a number of problems with parallel switching due to uneven sharing of the load current during conduction and during switching.
For higher performance, including better regulation, smaller size, lower weight, faster response, and, up to a point, lower cost, a higher switching frequency is used. A higher switching frequency reduces the size of the magnetic components of the converters. Also, the high frequency makes it possible to regulate the output more quickly. A problem with high switching frequencies is that components get less efficient, thereby limiting the practicality of raising the switching frequency. A conventional approach to getting around this limitation is to use a resonate converter of some type. Resonate converters, in general, however, add to the complexity of the circuits. They tend to have more limited operating ranges and other performance limits, but in some cases are very small and efficient.
Implementing multiphase converters is another approach to avoiding the limitations of losses due to increased switching frequencies in simple pulse-width modulation (PWM), hard-switched converters. FIG. 4B shows a simple prior art multiphase converter circuit 440. The advantages of multiphase converter circuits, such as circuit 440, have been established in some applications, like the voltage-regulation module (VRM) concepts of Dr. Fred C. Lee at the University of North Carolina. Circuit 440 of FIG. 4B is basically the circuit used by Dr. Lee in his VRM concepts. Advantages of circuit 440 include high bandwidth with lower effective switching frequency, lower ripple current in the DC bus capacitance, higher current capability and smaller size. Circuit 440 is simply one of a whole class of multiphase converters. In general, all multiphase converters are made of either a number of basic buck/boost or half-bridge switching cells, for example, basic buck/boost cells 444A-B of FIG. 4B, arranged in parallel with one another. The switching cells are switched at differing switching times relative to one another but with the same duty cycle and frequency at any instant in time. However, the switching among the cells are phase shifted from each other in various ways.
There are a number of shortcomings associated with conventional multiphase converters. These shortcomings include: their magnetic components are needed to prevent circulating AC currents; their magnetic circuits are typically expensive and complex to design; current balance between the switching cells at low frequency is a problem; control methods are not well established; and switch timing can be complex. The present invention includes features that address all these issues.